1. Field of the Invention
The present invention relates to pseudolite transmitters, or more specifically, to an integrated split spectrum pseudolite/satellite base station transmitter.
2. Discussion of the Prior Art
The Global Positioning System (GPS) is a system of satellite signal transmitters that transmits information from which an observer""s present location and/or the time of observation can be determined. Another satellite-based navigation system is called the Global Orbiting Navigational System (GLONASS), which can operate as an alternative or supplemental system.
The GPS was developed by the United States Department of Defense (DOD) under its NAVSTAR satellite program. A fully operational GPS includes more than 21 Earth orbiting satellites approximately uniformly dispersed around six circular orbits with four satellites each, the orbits being inclined at an angle of 55xc2x0 relative to the equator and being separated from each other by multiples of 60xc2x0 longitude. The orbits have radii of 26,560 kilometers and are approximately circular. The orbits are non-geosynchronous, with 0.5 sidereal day (11.967 hours) orbital time intervals, so that the satellites move with time relative to the Earth below. Generally, four or more GPS satellites will be visible from most points on the Earth""s surface, and can be used to determine an observer""s position anywhere on the Earth""s surface, 24 hours per day. Each satellite carries a cesium or rubidium atomic clock to provide timing information for the signals transmitted by the satellites. An internal clock correction is provided for each satellite clock.
Each GPS satellite continuously transmits two spread spectrum, L-band carrier signals: an L1 signal having a frequency f1=1575.42 MHz (nineteen centimeter carrier wavelength) and an L2 signal having a frequency f2=1227.6 MHz (twenty-four centimeter carrier wavelength). GPS satellites transmit both a C/A code and a P-code. There are a total of 32 pseudo random (PRN) C/A codes, with each satellite generating a different C/A code. The code modulations that produce either a P-code or a C/A code are impressed onto the L1 carrier and the L2 carrier.
The deployment of additional frequencies is being planned by the DOD. More specifically, DOD is exploring several options to maintain, or improve, the performance of civilian applications of GPS without compromising military utilities. Indeed, the civilian community does not have a second frequency. Today, corrections are based upon L2, which is a military frequency, and subject to DOD use and control. The addition of L5 to the GPS constellation on the Block HF satellites would, at a minimum, assure the civilian community of the existence of reliable dual frequency transmissions.
As a result, a new GPS frequency, L5, is being considered for civil sector uses in order to reserve L2 for military purposes. This new frequency is targeted to provide both carrier phase and C/A-code range information. Two frequencies are proposed for L5; the first being 1207 MHz yielding a 368 MHz separation from L1, and the second being 1309 MHz having a separation of 266 MHz from
The GPS satellite bit stream includes navigational information on the ephemeris of the transmitting GPS satellite (which includes orbital information about the transmitting satellite within next several hours of transmission) and an almanac for all GPS satellites (which includes a less detailed orbital information about all other satellites). The transmitted satellite information also includes parameters providing corrections for ionospheric signal propagation delays (suitable for single frequency receivers) and for an offset time between satellite clock time and true GPS time. The navigational information is transmitted at a rate of 50 Baud.
A second satellite-based navigation system is the Global Orbiting Navigation Satellite System (GLONASS), placed in orbit by the former Soviet Union and now maintained by the Russian Republic. GLONASS uses 24 satellites, distributed approximately uniformly in three orbital planes of eight satellites each. Each orbital plane has a nominal inclination of 64.8xc2x0 relative to the equator, and the three orbital planes are separated from each other by multiples of 120xc2x0 longitude. The GLONASS satellites have circular orbits with a radii of about 25,510 kilometers and a satellite period of revolution of 8/17 of a sidereal day (11.26 hours). A GLONASS satellite and a GPS satellite will thus complete 17 and 16 revolutions, respectively, around the Earth every 8 days. The GLONASS system uses two carrier signals L1 and L2 with frequencies of f1=(1.602+9k/16) GHz and f2=(1.246+7k/16) GHz, where k (=1,2, . . . 24) is the channel or satellite number. These frequencies lie in two bands at 1.597-1.617 GHz (L1) and 1,240-1,260 GHz (L2). The L1 code is modulated by a C/A-code (chip rate=0.511 MHz) and by a P-code (chip rate=5.11 MHz). The L2 code is presently modulated only by the P-code. The GLONASS satellites also transmit navigational data at a rate of 50 Baud. Because the channel frequencies are distinguishable from each other, the P-code is the same, and the C/A-code is the same, for each satellite. The methods for receiving and demodulating the GLONASS signals are similar to the methods used for the GPS signals.
The European Union plans to develop by 2008 the system of navigation and positioning by satellite designed exclusively for civil purposes-the GALILEO system. GALILEO should enable each individual, by way of a small, cheap individual receiver, to know his or her position to within a few meters, with guaranteed continuity of transmission of the signal. The GALILEO project, supported by the European Space Agency, aims to launch a series satellites at around 20 000 km to be monitored by a network of ground control stations, in order to provide world cover. GALILEO system should be integrated into the existing GNSS-Global Navigation Satellite System, comprising at present time GPS and GLONASS satellite systems.
Reference to a Satellite Positioning System or SATPS herein refers to a Global Positioning System, to a Global Orbiting Navigation System, to a GALILEO project, and to any other compatible satellite-based system that provides information by which an observer""s position and the time of observation can be determined, all of which meet the requirements of the present invention.
A Satellite Positioning System (SATPS), such as the Global Positioning System (GPS) or the Global Orbiting Navigation Satellite System (GLONASS), uses transmission of coded radio signals, with the structure described above, from a plurality of Earth-orbiting satellites. A SATPS antenna receives SATPS signals from a plurality (preferably four or more) of SATPS satellites and passes these signals to an SATPS signal receiver/processor, which (1) identifies the SATPS satellite source for each SATPS signal, (2) determines the time at which each identified SATPS signal arrives at the antenna, and (3) determines the present location of the SATPS satellites.
The range (ri) between the location of the i-th SATPS satellite and the SATPS receiver is equal to the speed of light c times (xcex94ti), wherein (xcex94ti) is the time difference between the SATPS receiver""s clock and the time indicated by the satellite when it transmitted the relevant phase. However, the SATPS receiver has an inexpensive quartz clock which is not synchronized with respect to the much more stable and precise atomic clocks carried on board the satellites. Consequently, the SATPS receiver estimates a pseudo-range (pri) (not a true range) to each satellite.
After the SATPS receiver determines the coordinates of the i-th SATPS satellite by demodulating the transmitted ephemeris parameters, the SATPS receiver can obtain the solution of the set of the simultaneous equations for its unknown coordinates (x0, y0, z0) and for unknown time bias error (cb). The SATPS receiver can also determine velocity of a moving platform.
The given below discussion, (applicable to any satellite navigational system, but focused on GPS applications to be substantially specific) can be found in xe2x80x9cGlobal Positioning System: Theory and Applicationsxe2x80x9d, Volume II, Chapters 1 and 5, by Bradford W. Parkinson and James J. Spilker Jr., published by the American Institute of Aeronautics and Astronautics, Inc. in 1996.
Typically, GPS based positions are calculated using the World Geodetic System of 1984 (WGS84) coordinate system. These positions are expressed in Earth Centered Earth Fixed (ECEF) coordinates of X, Y, and Z axes. These positions are often transformed into latitude, longitude, and height relative to the WGS84 ellipsoid.
Differential Global Positioning System (DGPS) is a technique that significantly improves both the accuracy and the integrity of the Global Positioning System (GPS). The most common version of DGPS requires high-quality GPS xe2x80x9creference receiversxe2x80x9d at known, surveyed locations. The reference station estimates the slowly varying error components of each satellite range measurement and forms a correction for each GPS satellite in view. This correction is broadcast to all DGPS users on a convenient communication link. Typical ranges for a local area differential GPS (LADGPS) station are up to 150 km. Within this operating range, the differential correction greatly improves accuracy for all users, regardless of whether selective availability (SA) is activated or is not. This improvement in the accuracy of the Global Positioning System (GPS) is possible because the largest GPS errors vary slowly with time and are strongly correlated over distance. DGPS also significantly improves the xe2x80x9cintegrityxe2x80x9d of GPS for all classes of users, because it reduces the probability that a GPS user would suffer from an unacceptable position error attributable to an undetected system fault.
Most DGPS systems use a single reference station to develop a scalar correction to the code-phase measurement. If the correction is delivered within 10 seconds, and the user is within 1000 km, the user accuracy should be between 1 and 10 meters. Users with very stringent accuracy requirements may be able to use a technique called carrier-phase DGPS or CDPGS. These users measure the phase of the GPS carrier relative to the carrier phase at a reference site; thus achieving range measurement precision that is a few percent of the carrier wavelength, typically about one centimeter. These GPS phase comparisons are used for vehicle attitude determination and also in survey applications, where the antennas are separated by tens of kilometers. If the antennas are fixed, then the survey is called static, and millimeter accuracies are possible, because long averaging times can be used to combat random noise. If the antennas are moving, then the survey is kinematic, and shorter time constants should be used with some degradation of accuracy.
Pseudolites (PLs) are ground-based transmitters that can be configured to emit GPS-like signals for enhancing the GPS by providing increased accuracy, integrity, and availability. Accuracy improvement can occur because of better local geometry, as measured by a lower vertical dilution of precision (VDOP). Availability is increased because a PL provides an additional ranging source to augment the GPS constellation.
However, a potential user of PL ranging signals should address the xe2x80x9cnear-farxe2x80x9d problem associated with the PL signal level. One solution to the near-far problem is to configure a set of pseudolites operating within the GPS frequency bands (L1:1565-1585 MHz or L2:1217-1237 MHz) to serve a limited area with a power level low enough to preclude appreciable interference to standard GPS signals. Another solution to the near-far problem is to design the PL signal configuration to operate within L1 band and mitigate or virtually eliminate the near-far issue.
What is needed is a pseudolite transmitter having a designed signal configuration that allows to operate within L1 band and mitigate or virtually eliminate the near-far issue, integrated with a satellite base station (SBS) that allows to lock the timing of the pseudolite transmitter to the satellite time, and to provide an automatic determination of the location of the integrated pseudolite/satellite base station transmitter.
To address the shortcomings of the available art, the present invention provides an integrated split-spectrum pseudolite/satellite base station (SS-PL)/SBS transmitter that allows to lock the timing of the (SS-PL) ranging signals to the satellite time, and to provide an automatic determination of the location of the integrated split-spectrum pseudolite/satellite base station (SS-PL)/SBS system.
One aspect of the present invention is directed to an integrated split-spectrum pseudolite/satellite base station (SS-PL)/SBS transmitter comprising: (a) a satellite base station (SBS); and (b) a split-spectrum pseudolite (SS-PL) transmitter co-located with the SBS.
In the preferred embodiment, the split-spectrum pseudolite (SS-PL) generates a split-spectrum sideband signal that minimizes interference with the reception of at least one satellite signal by the SBS, and the SBS provides a timing synchronization signal, and provides a self-surveying capability for the (SS-PL).
In one embodiment of the present invention, the integrated split-spectrum pseudolite/satellite base station (SS-PL)/SBS transmitter includes a differential GPS base station configured to provide a GPS timing synchronization signal to the SS-PL transmitter, and configured to provide the self-surveying capability for the (SS-PL) transmitter with a sub-meter accuracy. In another embodiment of the present invention, the integrated split-spectrum pseudolite/satellite base station (SS-PL)/SBS transmitter comprises an RTK GPS base station configured to provide a GPS timing synchronization signal to the split-spectrum (SS-PL) transmitter, and configured to provide the self-surveying capability for the split-spectrum pseudolite (SS-PL) with a centimeter accuracy.
In the preferred embodiment of the present invention, the integrated split-spectrum pseudolite (SS-PL)/RTK GPS base station transmitter configured to transmit position determining split-spectrum L1 GPS comprises a code generator for generating a pseudolite P-code. In one embodiment, the integrated split-spectrum pseudolite (SS-PL)/RTK GPS base station transmitter further includes a modulator for modulating the code and the GPS navigation data to produce a modulated split-spectrum sideband L1 signal. In another embodiment, the integrated split-spectrum pseudolite (SS-PL)/RTK GPS base station transmitter further includes a modulator for modulating the code and the GPS navigation data to produce a modulated split-spectrum sideband L2 signal. In one additional embodiment, the integrated split-spectrum pseudolite (SS-PL)/RTK GPS base station transmitter further includes a modulator for modulating the code and the GPS navigation data to produce a modulated split-spectrum sideband L5 signal.
In one embodiment, the pseudolite P-code is delayed as compared with a GPS P-code by a delay D, and the delay D is used to identify the integrated split-spectrum pseudolite/satellite base station (SS-PL)/RTK GPS base station transmitter as a (SS-PL)/RTK GPSD ntransmitter having the D-identification number.
In one embodiment, the integrated split-spectrum pseudolite (SS-PL)/RTK GPS base station transmitter further includes: a GPS receiver configured for receiving a GPS satellite synchronization signal and for receiving a plurality of exact GPS satellite frequencies; a radio receiver for receiving a set of GPS formatted navigational data; a message generator for generating a set of (SS-PL) data message including a set of the (SS-PL) positional data responsive to the set of GPS formatted navigational data; a signal generator for generating a standard reference frequency; a pseudolite antenna for transmitting the position determining sideband L1, L2, or L5 split-spectrum signal; and a GPS receiver for detecting at least one additional integrated split-spectrum pseudolite (SS-PL)/RTK GPSD1 transmitter having a D1-identification delay number.
In one embodiment, the integrated split-spectrum pseudolite (SS-PL)/RTK GPS base station transmitter includes a modulator configured to modulate the code and the GPS navigation data to produce a modulated split-spectrum sideband L1, (or L2, or L5) signal having a peak power at frequencies at which P(Y) code has nulls. In another embodiment, the integrated split-spectrum pseudolite (SS-PL)/RTK GPS base station transmitter includes a modulator configured to modulate the code and the GPS navigation data to produce a modulated split-spectrum sideband (L1∓xcex941) MHz signal having a frequency shift xcex941 ( or (L2∓xcex942) MHz signal having a frequency shift xcex942, or (L5∓xcex945) MHz signal having a frequency shift xcex945), and having a peak power at frequencies at which P(Y) code has nulls.
Another aspect of the present invention is directed to a method for generating a split-spectrum sideband signal by an integrated split-spectrum pseudolite/satellite base station (SS-PL)/GPSD transmitter having a delay D identification number.
In one embodiment, the method of the present invention comprises the following steps: (1) providing a timing synchronization signal by the satellite base station; (2) providing a self-surveying capability for the split-spectrum pseudolite (SS-PL) by the satellite base station; and (3) generating a split-spectrum sideband signal by the integrated split-spectrum pseudolite/satellite base station (SS-PL)/GPSD transmitter. The split-spectrum sideband signal includes a set of position coordinates for the integrated split-spectrum pseudolite/satellite base station (SS-PL)/GPSD transmitter having the delay D identification number.
In the preferred embodiment, the method of the present invention further includes the steps of: (4) detecting at least one additional split spectrum sideband signal generated by at least one additional active integrated split-spectrum pseudolite/satellite base (SS-PL)/GPSD* transmitter having a delay D* identification number; (5) determining a set of coordinates for each integrated split-spectrum pseudolite/satellite base station (SS-PL)/GPSD* transmitter having the delay D* identification number; (6) matching each set of coordinates with one integrated split-spectrum pseudolite/satellite base station (SS-PL)/GPSD* transmitter; and (7) creating a database including a set of coordinates and an identification number D* for each integrated split-spectrum pseudolite/satellite base station (SS-PL)/GPSD* transmitter. In one embodiment, a message including the database including the set of coordinates and the identification number D* for each active integrated split-spectrum pseudolite/satellite base station (SS-PL)/GPSD* transmitter is broadcasted.